1. Field of the Invention
The present invention relates to a process for enhanced recovery of hydrocarbons, and more particularly to a process for enhanced recovery of oil from heterogeneous reservoirs.
2. Description of Related Art
Use of conventional enhanced oil recovery processes, such as water flooding and gas flooding, in reservoirs with heterogeneous permeability results in flooding fluids channeling through higher permeability layers and bypassing lower permeability layers. Thus, significant quantities of oil remain in the lower permeability layers.
One approach to increasing the oil recovery from the lower permeability layers is a foam matrix conformance treatment, in which a foam is placed in the higher permeability layers of the heterogeneous formation to divert the flooding fluid through the lower permeability layers. Such areal conformance treatments are limited in their applicability to regions within about 30 to 40 feet of the injection wellbore. Thus, foam conformance treatments are generally not appropriate for applications where the injection and production wells are far apart, as, for example, in most offshore operations. Further, oil in the reservoir emulsifies the surfactant in the foam and breaks the foam down.
Cyclic wettability alteration, described in U.S. Pat. No. 4,842,065 to D. C. McClure, is an approach to increasing the recovery of oil from oil-wet fractured formations; the reservoir is flooded cyclically with a surfactant solution and water. The surfactant solution enters the fracture network of the formation and subsequently imbibes into a portion of the matrix blocks adjacent to the fracture network. The solution renders the portion of the matrix blocks it contacts surfactant-wet and simultaneously displaces oil from the surfactant-wet portion of the matrix blocks into the fracture network. Following injection of the surfactant solution, the formation is water flooded. The water flood desorbs the water soluble fraction of the surfactant mixture from the surfactant-wet portion of the matrix blocks, restoring this portion of the matrix blocks to an oil-wet condition. The water flood also sweeps oil from the fracture network to the surface, where the oil is recovered via a production well. The cycle can be repeated as often as desired.
Another approach to increasing oil recovery from lower permeability layers is water-alternating-gas (WAG)injections. U.S. Pat. No. 4,846,276 to H. K. Haines describes a WAG process in which gas is injected into an oil-bearing formation. The gas injection pressure is less than the formation fracturing pressure and the minimum miscibility pressure of the injection gas in the oil in place, but greater than the bubble point pressure of the oil. When oil production has declined to a predetermined level, gas injection is terminated. Water injection is then initiated from an injection well while maintaining the production well in operation; oil is continuously produced from the production well simultaneously with water injection until oil production diminishes to a predetermined level. The injection cycle may then be repeated.
One speculated mechanism for the WAG process relies on the ability of the injected gas to reduce the viscosity and density of the oil in place by swelling the oil despite the relative immiscibility of the gas in the oil. The injected water subsequently sweeps more oil to the production well because the oil is less viscous and less dense.
Another possible mechanism for the WAG process is gas trapping. Injected gas displaces water occupying pore spaces in the formation, and the gas subsequently occupies the space. When the formation is then flooded with water, the gas in place diverts the water to oil-bearing portions of the formation which have not been previously flooded. Thus, the gas flood effectively reduces the volume of the formation which the water flood must sweep to recover a given quantity of oil and promotes sweeping of pore spaces which would not otherwise be contacted by the water.
A third possible mechanism for oil mobilization during the WAG process is gravity segregation. The gas is significantly less dense than oil and water. As the gas moves upward through the formation rock, it displaces oil downward.
U.S. Pat. No. 4,113,011 to G. G. Bernard describes another process for enhanced oil recovery using fluid displacement by dense carbon dioxide in situations where foam formation is unsatisfactory. For example, many foaming agents are unstable in acidic and highly saline media. In the process, a slug of an aqueous surfactant solution is injected into the formation. The preferred surfactant solution contains aqueous alkyl polyethylene oxide sulfate. The surfactant solution is then displaced by a subsequently injected quantity of carbon dioxide dense fluid. The carbon dioxide pressure is greater that 1500 psi, above the miscible-displacement pressure but below the formation fracture pressure. An optional drive fluid, such as water or brine, can be injected to drive the carbon dioxide through the reservoir.
The use of high pressure and dense carbon dioxide results in a smaller density difference between the oil and the flooding phase, thereby decreasing the amount by which the oil expands when contacted by the carbon dioxide. The use of dense carbon dioxide also eliminates gas trapping as a mechanism for diverting aqueous fluids into previously unflooded oil-bearing portions of the formation. Thus, the use of dense carbon dioxide reduces the beneficial effects obtained when a gas phase is used, as in the WAG process.
In addition, liquefied carbon dioxide is also less available and more expensive than other gases for injection, such as produced natural gas. Other advantages from using natural gas include safer operation, the ability to use smaller gas compressors, and a reduced risk of undesirable formation fracturing. Natural gas does not liquefy under reasonable injection pressures and reservoir temperatures.
As previously mentioned, neither the WAG process nor the cyclic wettability process results in a complete sweep of the formation. Although both of these processes reduce the tendency for water fingering during enhanced recovery in a heterogeneous formation, neither completely eliminates fingering. Thus, there is a need for a process which further increases volumetric sweep efficiency so as to increase the quantity of oil that can be recovered during enhanced recovery. Additionally, for reasons set forth above, a need exists for an enhanced recovery process that combines the benefits of the WAG and cyclic wettability alteration processes, using a broad range of gas compositions at a broad range of injection pressures.
Accordingly, it is an object to the present invention to provide a method which improves volumetric sweep efficiency and increases oil recovery. It is a further object of the present invention to combine the effects of the WAG and cyclic wettability processes.